Turbofan engines typically have a fan with a hub and a plurality of fan blades disposed for rotation about a central axis. The casing surrounding the fan blades must be able to contain a broken fan blade propelled outwardly from the rotating hub at high speed.
Thus, the fan case includes a containment structure, which may have one of many various known designs, including designs employing composites, which can include a containment fabric layer, such as Kevlar®. The containment fabric is typically wrapped in multiple layers around a relatively thin, often penetrable supporting case, positioned between the blades and the fabric layer. Thus, a released blade will penetrate the support case and strike the fabric. The fabric deflects radially capturing and containing the released blade but largely remains intact.
One problem with such arrangement is that a fan blade tip rub may ruin the containment fabric if the blade tip contacts the containment fabric, thereby prejudicing the strength of the fabric. For this reason, a larger tip clearance is usually provided between the blade tips and the fan case to ensure tip rubs do not occur. This however results in a less efficient fan, larger fan case envelope and thus in extra engine weight.
Accordingly, there is a need to provide an improved softwall fan case containment design.